Patent GB-1388617-A (GB)
Title: Distance measuring instrument
IPC: G01S17/32
Filed: 1972-03-08
Abstract: 1388617 Photo-electric distance measurement CUBIC CORP 8 March 1972 [8 March 1971] 10786/72 Heading H4D A reflection type range finder comprises means for amplitude modulating a light (or infra-red) beam with a frequency modulated carrier, means for coupling the outgoing light beam directly to the receiver to determine internal phase shifts and means for determining and utilizing the phase shifts in the carrier and modulating signals of the outgoing and incoming light beams to determine the distance of a reflecting object, e.g. a corner cube reflector as illustrated in Figs. 10-12 (not shown). Master oscillator 210, Fig. 14, of frequency f1 feeds phase detector 220 so that oscillator 212 of frequency f2 = 16f1 is synchronized to the master oscillator which via line 236 also feeds subcarrier frequencies f1/2 and f1/64 to frequency modulate oscillator 212, the resulting signal being fed to a light emitting diode 30 (also see Fig. 2, not shown). In the receiver, Fig. 15, a second oscillator 262 of frequency f2 + 8À932 KHz is synchronized to the master oscillator 210 and dividers 266, 268 supply subcarriers f1/2+279À12 Hz and f1/64+8À722 Hz to phase modulate the output from oscillator 262 so that when the received signals are mixed with signals from line 257 a carrier of 8À932 KHz amplitude modulated by 279À12 and 8À722 Hz subcarriers is obtained. These signals are separated by filters 251, 253, 255 respectively. Units 246, 291, 293 and 295 of Fig. 14 provide reference signals at these frequencies, locked to the transmitted signals so that the phase shifts may be determined. The received data signals and reference signals are fed to the circuit of Fig. 16. Programmer 304 sequentially selects data and reference signals of each frequency, together with clock pulses of corresponding frequency for driving the logic circuitry. Clock pulses are gated through line 332 during the time interval corresponding to the phase shift between reference and data signals. These pulses are divided by unit 311 and a number of samples equal to the number by which unit 311 divides the pulses are taken. In this way an average number of clock pulses corresponding to the phase shift is built up in counter 310. As the clock pulse frequency is different for each carrier and subcarrier frequency the divider is "programmable" i.e. the number by which it divides is variable so that for a given phase shift the same number of pulses are given regardless of clock pulse frequency. After the pulses have been counted in counter 310, programmer 304 sends a pulse to a motor 80 (see Figs. 3, 5, not shown). This motor rotates a fibre optic link (72) into the light beam path and thus directly couples the transmitter and receiver. The circuitry of Fig. 16 now gives the internal phase shift of the equipment. Counter 310 is now programmed to count down so that the internal phase shift, which represents an error, is removed from the count in counter 310. The final count therefore gives the true phase shift due to distance only. This process is repeated for each of the frequency pairs at the inputs of units 300, 302. The remaining circuitry obtains the best answer from the three indicated phase shifts and displays the distance at unit 316.